Filter screen

ABSTRACT

There is provided an improved filter medium characterized by a very fine mesh woven wire straining fabric supported on a relatively large open mesh grid having substantially planar faces and cohered to the grid by sintering and without the aid of adhesives.

United States Patent 11 1 1111 3,747,770

Zentis 1451 July 24,1973

1 1 FILTER SCREEN 2,617,466 5 1954 Lowe 210 4119 2,910,|83 10/1959 Hayes21014911 x [751 lnvemo McKean' 3,165,473 1/1965 Pall et a1 210/510 73 iZul-n Industries, Inc" Erie, p 3,426,910 2/1969 Wmzen 210/499 X [22]Filed: June 20, 1969 Appl. No.2 835,078

References Cited UNITED STATES PATENTS 2/ 1949 Boucher 210/499 X PrimaryExaminer-Samih N. Zaharna Assistant Examiner-F. F. CalvettiAttorney-McNenny, Farrington, Pearne and Gordon [57] ABSTRACT There isprovided an improved filter medium characterized by a very fine meshwoven wire straining fabric supported on a relatively large open meshgrid having substantially planar faces and cohered to the grid bysintering and without the aid of adhesives.

9 Claims, 6 Drawing Figures FILTER SCREEN BACKGROUND OF THE INVENTIONThe use of very fine micro mesh woven wire straining fabrics formed fromextremely fine stainless steel wire is well known. Reference may be hadto the US. Pat. to Boucher No. 2,462,604 which fully describes filteringscreens possessing these characteristics. The disclosure of that patentis incorporated herein by reference.

In the manufacture of filters for aqueous media, e.g. in sewagetreatment, water straining operations, etc., the prior art has utilizedlarge drums which are rotatable, the surface of which is provided withpanels of such very fine woven wire mesh straining fabrics as abovedescribed. Although this straining fabric can be treated in such a wayas to confer thereon remarkable rigidity, it has been found necessary toprovide a grid type support having macro openings therein for the screenfabric to minimize breakage of such screens due to flexure occasioned byalternate filtering and backwashing. This occurs on virtually eachrevolution of the drum supporting such filtering media. The provision ofsuch a grid type support is also necessitated by the possibility offabric breakage during servicing. Tools and parts are sometimesinadvertently dropped on the fabric causing such breakage. Prior effortsto support screening media of the type described upon a grid haveincluded cementing the screening fabric to the support grid with anadhesive composition, usually an organic material such as an epoxyresin. A workable structure can be produced in such a fashion, butdifficulty is encountered with loss of area due to excessive amounts ofadhesive utilized in preparing supported screening panels, and ultimatedegradation of the adhesive because of exposure to chemicals, elevatedtemperatures, plant and animal life frequently associated with theaqueous media treated thereby. These factors contribute to loss ofefficiency and a high replacement rate of filter panels with therelatively expensive woven wire screening fabrics above described. Also,fine mesh woven wire screening fabric has been supported on coarse meshwoven wire mesh, e.g. 1/16 inch to V4 inch openings and surfaces whichare not planar? as the term is used herein. However, this mode ofsupporting a wire fabric to provide a filter medium for water treatmenthas not proved satisfactory as will be pointed out below.

The present invention providesan improved filter medium characterized bya woven wire screening fabric cohered to a wide mesh supporting gridhaving substantially planar faces. The area of cohesion is coextensivewith the planar surface area of the grid. These media are fabricated byheating the elements in laminar relation to a sintering temperature forthe metal comositions involved and for a period of time sufficient tocause the surfaces of therespective layers to become intimately bondedtogether by cohesion as distinct from adhesion. In a preferredembodiment of the pres fine or micro-mesh straining medium which is notonly free from the limitations imposed by organic adhesives, but hasconferred thereon further desirable characteristics by virtue of thesintering operation. The woven wire screening fabric prior to sinteringis quite flexible much as woven cloth formed from organic fibers. Afterexposure to the sintering operation, the screen itself undergoessintering such that the warp and weft elements at their respectivejunctures become cohered to one another, and the flexibility of thescreening fabric is greatly reduced giving a structural rigidity snadstability to the open area which resists flexure normally caused by theflow of fluid in opposite directions through the screen on eachrevolution on a drum. The wire cloth becomes also puncture resistant,and relieves stress thereby improving corrosion resistance.

BRIEF STATEMENT OF INVENTION Briefly stated, the present invention is ina filter medium consisting of a woven wire screening fabric having microopenings therein cohered to a rigid supporting gridihaving macroopenings therein and having substantially planar faces. The cohesionbetween the two elements is secured by exposing the elements incontiguous laminar relation to temperatures sufficient to sinter themetals without melting the metals.

The term rigid is not used herein in its absolute sense. It contemplatesa member which, relative to the wire fabric, is stiff and is capable ofrendering the composite of the fabric and grid self-supporting. The termdoes not mean inflexible as will appear. Panels are prepared flat, andwhen applied to a drum body are sufficiently flexible to conform to theconfiguration of the outer surface.

The supporting grid is also characterized by substantially planar faces.This means tht the top and bottom surfaces of the grid members definingthe openings lie in common, preferably parallel, planes, respectively,throughout their extent. Accordingly, the contact between the fabric andthe grid is over a flat surface which is co-extensive with the peripheryof each opening. That area of the surface of the grid which is incohering contact with the fabric is substantially equal to thedifference between the overall. area of the grid diminished by the openarea of the grid. Thus suitable grids include perforated plates andflattened expanded metal grids. Excluded is woven wire mesh.

Woven wire mesh is unsatisfactory for certain purposes, particularlywater straining, because contact between a wire fabric and such a widemesh screen is virtually limited to point contact or isolated smallareas of contact. Flexure of the screen in use can destroy the adhesionor the screen itself at such points. Moreover, between the fabric andthe wire mesh, pockets are created which trap debris and make adequatewashing of a filter medium so constructed difficult and inefficient.When cohesion between the screen and grid is over an area fully boundingthe individual openings both these problems are greatly alleviated tothe end that filter medium life is extended and filtering efficiency ismaintained at a high level for the lifeofthe filter medium. The largemesh of the supporting grid is beneficial in reducing sites for algaegrowth which can lead to screen blinding and cessation of filteringaction.

The filter media of the present invention are formed by placing therigid screen containing macro openings and substantially planarfaces inlaminar relation with the woven wire screening fabric, applying a load,albeit relatively light, normal to the plane of the grid uniformly oversaid grid, and heating the composite to a temperature and for a periodof time sufficient to sinter the contacting metals including thejunctures between warp and weft elements without melting the same,cooling the composite and removing the filter medium from the pile.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of afilter medium in accordance with the present invention showing anexpanded metal grid having macro openings therein and cohered thereto, awoven wire straining fabric having micro openings.

FIG. 2 is a cross-sectional view through a filter medium of the presentinvention.

FIG. 3 is an end view on a very much enlarged scale showing a typicalsingle warp Dutch twill weave.

FIG. 4 is a plan view of a section of a typical stainless steel wirefabric of the single warp Dutch twill weave useful as the filter elementof the present invention.

FIG. 5 shows a portion of a typical laminar composite structure or pilefor insertion in a sintering furnace to produce the reinforced filtermedia of the present invention.

FIG. 6 shows a drum, the outer surface of which is covered with andformed from panels of the filter media'of the present invention.

DESCRIPTION OF THE DRAWINGS Referring now more particularly to FIGS. 1and 2, FIG. 1 shows in perspective a fragment of a filter medium of thepresent invention. FIG. 2 is a crosssectional view of the filter mediumshown in FIG. 1 as it appears in the plane indicated by the line 22 ofFIG. 1. The filter medium is composed of a micro screen or woven wirefabric 10 which is tightly cohered to a supporting grid 12 having macroopenings therein. A specific example of a woven wire fabric suitable foruse in accordance with this invention is AlSI type 316 stainless steelwire cloth. A bead 14 may be applied to the marginal edges ofarectangular filter medium member after sintering. A marginal bead 14 isnot essential to the utility of the filter medium for its intendedpurpose, and where the panels are relatively small, less than 4 ft. onan edge, there is no real need for marginal reinforcement. However inlarge size drums, e.g. 25 ft. in diameter, marginal beads 14 will befound desirable and panels up to 8 ft. or more on an edge may be made.In certain instances, H-shaped beads are used between panels to provideintermediately reinforced large panels.

The supporting grid is also metallic and preferably of the same or verysimilar chemical composition as the woven wire fabric. The openings inthe supporting grid are very large in relation to the openings in thescreening material and desirably have an area of more than 0.5 sq. in.and preferably at least 1 sq. in. In a preferred ment about thecircumference of a drum 16, such as is shown in FIG. 6.

FIGS. 3 and 4 show a single warp Dutch twill weave having 3,600 toupwards of 250,000 openings per sq. in. These openings are of the orderof magnitude of from 5 to 300 microns. In a preferred embodiment, theopenings are from 21 to 40 microns or smaller. In FIGS. 3 and 4 the warpfilaments 18 are filled by weft lines 20 in an over-two, under-two,staggered relationship. This weave is particularly suitable for use inthis invention.

The fine mesh straining fabric is continuously bonded to a stainlesssteel expanded metal sheet of the type above described by a sinteringprocess. For the stainless steels, a sintering temperature of from 2,l00to 2,200 F. is satisfactory. It has been found convenient to effectsintering of the rectangular panels, which normally range from 1 ft. by2 ft., up to 18 inches X 60 inches in a pile of the type illustrated inFIG. 5. The pile is composed of lamina built up from a base plate 19 ofdecarbonized steel 1 inch thick having superimposed thereon a layer ofrefractory fibers 21. The next layer is a grid layer 24 havingsuperimposed thereon and in direct contact therewith a similarlyconfigured rectangularly shaped woven wire fabric layer 22. In directcontact with the wire fabric layer is a thermally stable nonmetallicseparator, e.g. ceramic mat layer 26. Covering this assembly there is asteel plate 28 also of decarbonized steel which is about 0.060 inchthick. Steel plates 28 may be thicker, for example A inch to A inchthick if desired. This constitutes a first tier of lamina, and a secondtier is similarly built considering the aforesaid separator steel plate28 as the first member of the second tier. Thus ceramic fibers 30 arerandomly disposed upon steel plate 28. Directly in contact with theceramic fiber layer 30 is a second grid layer 32, and the pile isthereafter repeated in tiers from three to nine or more times dependingupon the capacity of the furnace and the stability of the upper portionsof the pile against lateral movement. The top plate, not shown, may alsobe a 1 inch thick decarbonized steel plate. Weights may be applied tothe top plate for added pressure if desired. Normally it is convenientto tackweld the screen onto expanded metal grid at several points to aidin setting the pile in the furnace and preventing lateral displacementof the wire fabric with respect to the grid. A stack or pile is about 4inches high. It may be composed of a single tier, or a plurality oftiers and be as high as 18 inches or more if stability of the pile canbe maintained in the furnace. The furnace is maintained under ahydrogen, or reducing atmosphere, and is brought to sinteringtemperature in 6 to 8 hours and the pile held at this temperature (2,l50F.) for about 60 minutes. Thereafter, the pile is cooled slowly to 400F. to 500 F. and then it is removed from the furnace and cooled to roomtemperature. The entire heat treating cycle requires about 26 hours.

In this manner, the wire fabric is cohered to the supporting grid andintimately bonded thereto by interaction between the respectivelycontacting surfaces of the woven fiber fabric and the supporting grid.No adhesive is required and the bond which is obtained under theseconditions is extremely good. Moreover, at the points of contact betweenthe warp and weft wires within the fabric itself, sintering occurs and agreat increase in the rigidity of the fabric and dimensional stabilityis also experienced.

The advantage of this can be appreciated when it is realized that duringthe filtering operation, water flows through the filtering mediumpassing first through the screen and then through'the supporting gridwhich is on the outside of the drum 16. In normal usage of thesedevices, the filter medium is then submitted to a water backwash whichis spray applied, the water first passing through the grid 12 and thenthrough the wire fabric in the opposite direction to that normallyemployed in filtering. Thus, the screen in a filter drum is subjected toforces normal to the screen surface and moving in opposite radialdirections once for each rotation of the drum. Because of the sinteringprocedure, the screens are less subject to failure adjacent their pointof contact with the supporting grid than heretofore possible. Thecontact normally terminates along a sharp or abrupt edge of the grid,and flexure over such an edge induces rapid failure. With the sinteredfilter media of this invention, sintering imparts a rigidity to thewoven wire mesh and creates a cantilever effect in the fabric thatremoves the flexure point away from the sharp edge of the grid and thusminimizes fatigue and failure of the medium.

Referring now more particularly to FIG. 6, there is here shown a drum 16having secured to the outer periphery thereof a plurality of filterelements 34. The drum itself is composed of end rings such as end ring36 joined to each other by longitudinal members 38. Circumferentialmembers, not shown, may be utilized when the filter drum is longer than,for example, three feet. The longitudinal members 38 provide a means forsecuring the filter media 34 to the drum surface, and to this endcircumferential segments 40 at each end ring 36 may be bolted to the endring, and longitudinal retaining bars 42 bolted to the longitudinalmembers 38. The devices of the present invention may be fabricated withmarginal beads 14 to protect the marginal edges of the filter media 34and also to coact with the retaining bars 42 and the circumferentialsegments 40 for securing the filter media 34 to the drum 16. Eachmarginal bead is clamped between a member 38 and a bar 42 so that thebolts do not pass through the screening material. Often it is convenientto interpose between the edge of the filter medium 34 and the drum bars42 and segments 40, a closed cell vinyl adhesive tape about is inchthick by 0.75 inch wide as a sealing member. Should one of the filtermedia 34 be damaged in use, it is a relatively simple matter to removethe retaining bars and circumferential segments and to replace thedefective member with a new member. By securing the filter media 34 inthe manner stated, passage of the securing means or bolts through thescreen itself is avoided whereby piercing of the screen and subsequentintroduction of points of weakness is obviated. The built-up drum 16 isthen mounted in a conventional manner to receive and strain an aqueoussystem.

There has thus been provided an improved filter medium composed of afine woven wire fabric having micro openings, i.e., openings of theorder of 5 -300 microns. This fabric is cohered to and supported by agridwhich is characterized by macro openings, i.e., of the order of 0.5to 2 sq. in. in size. Cohesion or diffusion bonding of the fabric to thegrid is effected by sintering.

What is claimed is:

l. A filter medium consisting of a. a woven metallic wire screeningfabric having micro openings therein, the size of the openings being inthe range of from 5 to 300 microns, in sintered cohered relation to b. arigid supporting grid of flattened expanded metal having macro openingstherein, said openings having closed peripheries and thecenter-to-center dimension of the openings being from 0.5 to 2 inches,and said grid having substantially planar faces, the metals of saidscreening fabric and said supporting grid being of an equivalentchemical composition and wherein the area of cohesion is coextensivewith the confronting surface area of the grid.

2. A filter medium in accordance with claim 1 wherein the screeningfabric is stainless steel.

3. A filter medium in accordance with claim 1 wherein the fabric isthermally cohered to said supporting grid.

4. A filter medium in accordance with claim 1 which is rectangularlyconfigured.

5. A filter medium in accordance with claim 1 also including a marginalreinforcing bead along its edges.

6. A filter medium in accordance with claim 5 in which the marginal beadis a U-shaped channel.

7. A filter medium in accordance with claim 1 wherein the woven wirescreening fabric is a Dutch twill weave of stainless steel havingupwards of 3,600 to 250,000 openings per square inch.

8. In a water straining apparatus including a drum body, the improvementwhich comprises a filtering surface on said drum body formed from aplurality of adjacently disposed panels each consisting of:

a. a woven metallic wire screening fabric having micro openings therein,the size of the openings being in the range of from 5 to 300 microns, insintered cohered relation to b. a rigid supporting grid of flattenedexpanded metal having macro openings therein, said openings havingclosed peripheries and the center-to-center dimension of the openingsbeing from 0.5 to 2 inches, and said grid having substantially planarfaces, and

c. means coacting with the marginal edges of said panels for securingsaid panels to said drum body, the metals of said screening fabric andsaid supporting grid being of an equivalent chemical composition andwherein the area of cohesion is coextensive with the confronting surfacearea of the grid.

9. A water straining apparatus comprising in combination a cylindricaldrum body and a filtering surface on said drum body, said filteringsurface being formed from a plurality of adjacently disposed filtermedia panels covering the cylindrical surface of said drum body and eachconsisting of:

0. means coacting with the marginal edges of saidv panels for securingsaid panels: to said drum body, the metals of said screening fabric andsaid supporting grid being of an equivalent chemical composition andwherein the area of cohesion is coextensive with the confronting surfacearea of the grid.

3,747,770 Dated July 24., 1973 Inventorg) Ramon J. Zentis It iscertified that error appears in the above-identified patent o and thatsaid Letters Patent are hereby correct ed as shown below:

i EDWARD M.FLETCI-IER,JR.

Attesting Officer Columh 2, J .'ine ll, "snad should be --and,- and line36, Y'tht" should be -th 1t- COllJmn 4, line 31," "1 4 inch thick"should be --1 2 inch thick--.

Signed and sealed this 25th day of December 1973.

(SEAL) Attest:

' RENE D. TEGTMEYER Acting Commissioner of Patents

1. A filter medium consisting of a. a woven metallic wire screeningfabric having micro openings therein, the size of the openings being inthe range of from 5 to 300 microns, in sintered cohered relation to b. arigid supporting grid of flattened expanded metal having macro openingstherein, said openings having closed peripheries and thecenter-to-center dimension of the openings being from 0.5 to 2 inches,and said grid having substantially planar faces, the metals of saidscreening fabric and said supporting grid being of an equivalentchemical composition and wherein the area of cohesion is coextensivewith the confronting surface area of the grid.
 2. A filter medium inaccordance with claim 1 wherein the screening fabric is stainless steel.3. A filter medium in accordance with claim 1 wherein the fabric isthermally cohered to said supporting grid.
 4. A filter medium inaccordance with claim 1 which is rectangularly configured.
 5. A filtermedium in accordance with claim 1 also including a marginal reinforcingbead along its edges.
 6. A filter medium in accordance with claim 5 inwhich the marginal bead is a U-shaped channel.
 7. A filter medium inaccordance with claim 1 wherein the woven wire screening fabric is aDutch twill weave of stainless steel having upwards of 3,600 to 250,000openings per square inch.
 8. In a water straining apparatus including adrum body, the improvement which comprises a filtering surface on saiddrum body formed from a plurality of adjacently disposed panels eachcOnsisting of: a. a woven metallic wire screening fabric having microopenings therein, the size of the openings being in the range of from 5to 300 microns, in sintered cohered relation to b. a rigid supportinggrid of flattened expanded metal having macro openings therein, saidopenings having closed peripheries and the center-to-center dimension ofthe openings being from 0.5 to 2 inches, and said grid havingsubstantially planar faces, and c. means coacting with the marginaledges of said panels for securing said panels to said drum body, themetals of said screening fabric and said supporting grid being of anequivalent chemical composition and wherein the area of cohesion iscoextensive with the confronting surface area of the grid.
 9. A waterstraining apparatus comprising in combination a cylindrical drum bodyand a filtering surface on said drum body, said filtering surface beingformed from a plurality of adjacently disposed filter media panelscovering the cylindrical surface of said drum body and each consistingof: a. a woven metallic wire screening fabric having micro openingstherein, the size of the openings being in the range of from 5 to 300microns, in sintered cohered relation to b. a rigid supporting grid offlattened expanded metal having macro openings therein, said openingshaving closed peripheries and the center-to-center dimension of theopenings being from 0.5 to 2 inches, and said grid having substantiallyplanar faces, and c. means coacting with the marginal edges of saidpanels for securing said panels to said drum body, the metals of saidscreening fabric and said supporting grid being of an equivalentchemical composition and wherein the area of cohesion is coextensivewith the confronting surface area of the grid.